The United States, France and Britain are finalizing plans for an expected strike against the regime of Syrian President Bashar al-Assad. They blame Damascus for the alleged chemical attack on the town of Douma in the devastated Eastern Ghouta suburb of the capital.

"Some things are simply inexcusable, beyond the pale and in the worst interest of not just the chemical weapons convention but of civilization itself," Mattis told the House Armed Services Committee.

He said a chemical attack had likely occurred and hoped inspectors would be in place within a week to prove it.

"I believe there was a chemical attack and we are looking for the actual evidence," Mattis said.

World powers have threatened a strong response to Saturday's attack, with French President Emmanuel Macron saying Thursday he had "proof" Syria's government was behind it.

Syria and Russia have both denied the accusations, and the global chemical weapons watchdog said its team would begin its investigation on Saturday.

Lawmakers quizzed Mattis on what he made of President Donald Trump's muddled messaging over Syria.

Last week, Trump said he wanted to pull out of the war-torn nation, but this week he has taunted Assad ally Russia with boasts of an impending missile strike.

The recognition of Syria's complexities "means you are at times ... going to see contrary impulses," Mattis said.

Mattis insisted that Trump has not yet made a decision to strike Syria, after the president earlier had tweeted that action "could be very soon or not so soon at all!"

The Pentagon chief and other top security officials were due to head to the National Security Council later Thursday, where he said he would present "various options to the president."

"We're trying to stop the murder of innocent people. But on a strategic level, it's (about) how do we keep this from escalating out of control," Mattis said.

- Buildup -

Since Saturday's attack in Douma, there has been a sustained military buildup in the eastern Mediterranean.

A French frigate, British Royal Navy submarines laden with cruise missiles and the USS Donald Cook, an American destroyer equipped with Tomahawk cruise missiles, have all moved into striking range.

Several lawmakers grilled the Pentagon chief on the legal authorities the military has for action in Syria beyond the remit of its current mission, which is to work to destroy the Islamic State group.

The Pentagon is currently working under war powers granted in the aftermath of the September 11, 2001 attacks, even though operations across the Middle East have morphed broadly since those early days of seeking to destroy Al-Qaeda.

Mattis said a strike on the Assad regime in Syria would be authorized under Constitutional powers granted to the president, and would not require Congressional permission.

But Democratic Congresswoman Tulsi Gabbard disagreed, and said last year's strike on a Syrian airfield following a chemical attack in Khan Sheikhun was illegal.

"This is a complex area, I'll be the first to admit it," Mattis said.